1. Field of the Invention
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an optical alignment composition, an alignment layer, and an LCD having the alignment layer formed using the optical alignment composition.
2. Description of the Related Art
In general, a liquid crystal molecule has the characteristics of an intermediate state between a liquid and a solid, having the fluidity of a liquid and having the optical property of a solid, and can be changed in its optical anisotropy by an electric field or heat. A liquid crystal display device (LCD) utilizes such properties and is typically used as a flat-panel display device.
FIG. 1 is a schematic section view of a general LCD, in which transparent electrodes 3 and 3' made of indium tin oxide (ITO) are formed on a pair of upper and lower substrates 2 and 2' made of glass. Insulating layers 4 and 4' and alignment layers 5 and 5' for aligning liquid crystals are sequentially formed on the transparent electrodes 3 and 3'. A spacer 6 is sustained between the alignment layers 5 and 5' with a cell gap. A liquid crystal material is injected into the cell gap to form a liquid crystal layer 7. Polarizing plates 1 and 1' for polarizing entering and transmitted lights are provided at the exterior of the substrates 2 and 2'.
FIG. 2 shows the positional relationship between liquid crystal molecules and an alignment surface (substrate), in which a pretilt angle means an angle formed by the substrate surface and a liquid crystal director. An alignment layer must be formed by employing an appropriate alignment material or using an appropriate alignment method so that the pretilt angle of the liquid crystal can be adjusted within a proper range, which is essential for improving the display characteristic of the LCD.
For the purpose of forming an alignment layer, a rubbing treatment has been generally used, that is, a polymer resin film such as a polyimide film is formed on a substrate where an electrode is formed and then the film is rubbed with a piece of cloth. However, according to this method, dust or static electricity may be generated due to mechanical contact between the rubbing cloth and the alignment layer.
Thus, an optical alignment technology has recently been developed, which is non-destructive unlike the rubbing treatment. According to the optical alignment technology, polarized light is irradiated into a photopolymeric alignment layer and anisotropic photopolymerization occurs so that liquid crystal molecules are uniformly aligned.
As the material of a photopolymeric alignment layer, a polycinnamate such as PVCN (polyvinylcinnamate) or PVMC (polyvinyl methoxycinnamate) has been used. However, such polymers have poor thermal stability and it is difficult to control the pretilt angle of liquid crystals. Further, the pretilt angle is no more than 0.degree.. Thus, it is difficult to apply such polymers to the LCD. Although much research into methods for increasing the pretilt angle has been conducted, the result of the research has not been satisfactory.